1. Field of the Invention
The present invention relates to can combustors. In particular, the present invention relates to impingement cooled can combustors for gas turbine engines.
2. Description of the Related Art
Gas turbine combustion systems utilizing can type combustors are often prone to air flow mal-distribution. The problems caused by such anomalies are of particular concern in the development of low NOx systems. The achievement of low levels of oxides of nitrogen in combustors is closely related to flame temperature and its variation through the early parts of the reaction zone. Flame temperature is a function of the effective fuel-air ratio in the reaction zone which depends on the applied fuel-air ratio and the degree of mixing achieved before the flame front. These factors are obviously influenced by the local application of fuel and associated air and the effectiveness of mixing. Uniform application of fuel typically is under control in well designed injection systems but the local variation of air flow is often not, unless special consideration is given to correct mal-distribution.
The achievement of current levels of oxides of nitrogen set by regulations in some areas of the world calls for effective fuel-air ratio to be controlled to low standard deviations on the order of 10%. The cost of development of such combustion systems is high but can be significantly influenced by the right choice of configuration. Manufacturers of gas turbines have different approaches to the configurations which appear straight-forward but often find development troublesome and costly. To further illustrate these facts the configuration in FIG. 1, a schematic of a known impingement cooled can combustor, may be usefully discussed.
As schematically depicted in FIG. 1, can combustor 10 includes housing 12, an inner combustor liner 14, defining a combustion zone 16 and a dilution zone 18, as would be understood by those skilled in the art. Additionally, prior art combustor 10 includes a sleeve 20 having impingement cooling orifices 22 for directing cooling air against the outside surface of liner 14. Combustor 10 is configured to use dilution air for the cooling air, prior to admitting the dilution air to the dilution zone 18 through dilution ports 24. Air for combustion flows along passage 26 directly to swirl vanes 28 where it is mixed with fuel and admitted to combustion zone 16, to undergo combustion. Also depicted in FIG. 1 is a recirculation zone or pattern 32 that is established by the swirling air/fuel mixture and the can component geometry, to stabilize combustion.
The type of configuration shown in FIG. 1 may be used in a simple low NOx combustor where impingement cooling is preferred to that of film cooling. Generally, the use of film cooling in these low flame temperature combustors generates high levels of carbon monoxide emissions. External impingement cooling of the flame tube (liner) can curtail such high levels. The feature that appears initially attractive in the illustrated configuration is the additional use of the impingement air for dilution. However, in systems where high exit temperature is a performance requirement in addition to low NOx, the swirler/reaction zone air flow is a large proportion of total air flow and therefore cooling and dilution air flows are limited. Hence there is considerable advantage in combining these flows to optimize the overall flow conditions. Whereas the aerodynamics would seem to be satisfactory it should be seen that the swirler/reaction zone air flow is open to the effects of any mal-distribution that may be inherent in the incoming flow, namely in air passage 26. The effects of such mal-distribution on swirler/reaction zone fuel-air ratio and NOx are further amplified when the overall pressure loss of the combustor is required to be low.